The present project is designed to explore mechanisms of Na+ and K+ transport in mammalian cortical collecting tubule (CCT), and the modification of these processes by steroid hormones and arginine vasopressin (ADH). The technique of in vitro perfusion of rat CCT will be used to address the following problems: 1) The mechanism by which ADH stimulates Na+ transport. 2) The effect of ADH on K+ secretion and its relationship to Na+ absorption. 3) The effect of prostaglandin E2 on the CCT and on the actions of ADH. 4) The influence of steroid hormones on the ADH-responsiveness of the CCT. 5) The influence of ADH on the steroid-responsiveness of the CCT. 6) Development of mathematical models in connection with the above studies. The NRTC provides in ideal environment for basic laboratory investigation. The NRTC consists of 6 research laboratories and an extensive clinical service; the faculty has 6 Ph.D. and 9 M.D. members, most of which are independently funded and working on a variety of basic and clinical research projects. The NRTC has excellent resources and provides a forum for frequent interaction between its members.